This research involves the development of high performance liquid chromatography (HPLC) and its application to problems of biomedical and clinical significance. Our efforts involve the understanding of retention and high selectivity in HPLC. For this work we have developed a general theory of retention based on an expanded solubility parameter approach. This scheme provides for classification of solvents and adsorbents and allows predictions of selectivity. In a second area we are developing phase systems for high performance separation of proteins. This effort involves small particle diameter supports designed to provide separations based on exclusion and hydrophobic effects. In a third area, we have developed an analytical method based on HPLC for procainamide and N-acetyl procainamide in serum. Other applications include vitamin B6 analysis and determination of steroid hormones in serum at the trace level. Finally, we have validated our methodology for the analysis of free (i.e. unbound) thyroid hormone fractions using equilibrium dialysis and gas chromatography with electron capture detection. We have found a high correlation between our method and that of a standard procedure on euthyroid and pathological sera. As detection limits are as low as 0.2 pg, we have been able to observe free reverse T3 in one ml of euthyroid serum (approximately 1 pg).